Superheterodyne receiver



June 20, 1939.

D. E. FosTx-:Rv ET AL SUPERHETERODYNE RECEIVER Filed Jan. 4, 1938 BY CHA/2l w l v ATTO RNEY Patente-cl June 20, 1939 UNITED STATES PATENT oFF-1ct:

assignors to Radio Corporation of America, a corporation of Delaware Application January 4,

2 Claims.

Our present invention relates to signal receiving systems, and more particularly to receiving systems of the superheterodyne receiver type employing a local oscillator constructed to have sub- 5 stantial frequency stability.

In receiving systems of the type employing automatic volume control (AVC hereinafter) it has been observed that thevk action of the control circuit causes a fluctuation in plate voltages supplied to tubes not under control. Thismay be ascribed to the fact that the AVC circuit functions to change the plate current flowing through the controlled signal transmission tubes, and thereby changes the value of the direct current vl5 voltage fed from the receiver power supply circuit to other circuits of the receiver. For eX- ample, in a superheterodyne receiver employed in the short wave band of 4 to 22 megacycles (mc.) the AVC circuit Will act to cause the local oscillator to drift from its adjusted operating frequency. In a receiver of this type a change in the plate potential of the local oscillator tube will cause a substantial change in the frequency of the oscillator network. At the high frequencies employed in the 6-22 mc. range, a considerable change in the oscillator frequency will cause the I. F. value to depart to an extent such that the desired signal carrier will be lost.

Accordingly, it may be stated that it is` one 30 of the main objects of our present invention to provide a method of receiving modulated signals while employing an AVC circuit; the receiving system employing the superheterodyne method of reception by utilizing a local voscillator network 5 which isconstruoted and arranged to have its established operating frequency independent of variations in the plate potential of the oscillator tube, and which variations may be caused by the action of the AVC circuit.

Another important object of the invention is to provide an oscillator network which has its input and output circuits constructed and arranged to render the operating frequency of the network substantially independent of variations of the 45 plate potential of the oscillator tube; the circuits employed in the network being such that Y when the plate potential varies opposed frequency variation tendencies render the oscillator frequency substantially independent vof the plate y 50 potential variation.

, Another objectY of our invention is to provide a superheterodyne receiver of the type employed to receive frequencies of the order of 0.5-22.0 mc., and which receiveralso utilizes an AVC circuit; 55 the receiver having itsI local oscillator designed 1938, Serial No. 183,244

(Cl. Z50-20) and more especially to provide such circuits so that they are not only durable and reliable 'in operation, but are economically manufactured and assembled. I

'I'he novel features which We believe to be characteristic of our invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and `method'of operation will best be understood by reference to the following description taken in connection with the drawing in which we have indicated diagrammatically a circuit organization whereby our invention maybe carried into effect. In the drawing:

Fig. 1 schematically shows a superheterodyne receiver embodying the present invention,

Fig. 2 graphically illustrates the characteristic of the local oscillator network employedin the circuit of Fig. 1.

Referring now yto Fig. 1, it will be observed that the receiving system shown is of the superf f heterodyne type, and it is to be understood that the networks thereof, with the exception of the present invention, may be purely conventional in construction. For example, the receiver can be receive signals in the broadcast band of 500 to 1500 k. c. and it can also be used to receive signals in the short Wave range of the order of 0.5-22 mc. The various circuit elements enabling the receiver to be changed fromV the broadcast band to the short Waverange have not been shown, since those skilledin the art are fully aware vof the nature of such circuit elements. For the purpose of the present application let it be assumed that the receiving circuit in Fig. 1 is arranged to receive signals in the aforementioned short Wave range. v The signal collector l may be of any desired construction; Ait maybe a grounded antenna circuit a radio frequency dis- A of the multi-range type so that it can be used to y tribution line; a loop collector, or a collector device usually employed on mobile structures such as automobiles.

The signalcollector feeds the collected signals to a radio frequency amplifier network 2, and

the latter may comprise one or more stages of radio frequency amplification. The amplified signals are fed to a first detector network 3, and the latter customarily employs a turnable input circuit. The numeral 4 denotes the variable tuning condenser which is usually employed to tune the input circuit of the radio frequency amplifiers and the first detector to a common signal carrier frequency; it will be understood that the numeral 4 denotes, in schematic fashion, the gang of variable tuning condensers disposed in such tunable input circuits. The first detector network'has impressed thereon, in any suitable manner, locally produced oscillations from a local oscillator network comprising tube 5.

The latter tube includes a control grid a cathode 'I and a plate 8. The tank circuit of the oscillator includes a coil 9 which has connected in shunt therewith a variable condenser I0. 'Ihe rotors of variable condenser I are arranged for mechanical uni-control adjustment. with the Vrotors of the Variable tuning condensers and Athe broken lines II designate such uni-control tion. The high potential end of coil 9 is connected to plate 8 through a condenser I I', the junction of the low potential terminals of coil 9 and condenser I0 being established at ground potential.

The cathode 'I is connected to a suitable tap I2 on coil 9, and the control grid 6 is reactively coupled to coil 9 through a path which includes the condenser I3 and the small tickler coil I4. The leak resistor I is connected between the grid side of condenser I3 and the cathode lead of tube 5. Coil I4 is magnetically coupled to coil '9; if desired, coils 9 and I4 may be a common a value of substantially 460 kc., assuming that the signal is operating in a signal range of 0.5 to 22 mc. The I. F. output of the first detector network 3 is impressed upon an I. F. amplifier I'I,

and it will be understood that the network Il may include one or more stages of I. F. amplification and that the I. F. amplier tubes will have input and output circuits' each tuned to the operating I. F. value.

The amplified I. F. energy is impressed upon a demodulator, or second detector, network I8, and the audio output of the demodulated I. F. energy is impressed upon one or more stages of audio frequency amplication, followed byan audio reproducer. The numeral 20 denotes the receiver power supply network, and the latter is schematically represented since those skilled in the art are fully aware of its construction. The receiver powerv supply network is shown arranged to feed direct current to the plate circuit of the oscillator tube 5, and it will be understood that the plate 8 of the oscillator tube is connected to a point of proper positive potential on the power supply bleeder resistor through the resistor 2l. .The lead 22 denotes the direct current voltage connections from the supply network 20 to the various plate circuits of the networks 2, 3, I'I and I8. Of course, the supply network 20 usually comprises, if an A. C. source is used, a rectifier followed by a filter circuit, and a direct current voltage supply bleeder resistor being connected across the filter circuit output. The various connections 22 and 2| are usually made to a common direct current voltage supply bleeder resistor; a change in potential at the plates of tubesl used in networks 2, 3 and I'I will cause a change in the potential of plate 8 of tube 5. Such plate potential variations will occur, for example, when AVC is utilized to minimize fading effects.

Elects other than AVC action may cause the plate potential of oscillator 5 to vary; these are variation of line voltage, and iiuctuations due to variation of the plate current of the power tube during the audio frequency cycle. The source of all direct current potential is the rectifier tube; the rectifier itself, filter chokes, speaker fields etc., all, having some resistance, are interposed between the Voltage source and the amplilier and oscillator tubes. This resistance may be as little as 500 ohms, but is usually larger and may be much greater in some circuits. Change of the Voltage drop across such resistance occurs when the amplifier tube current (or power tube current at A. F.) Varies, producing corresponding changes in the plate potential of the oscillator.

The AVC circuit employed herein is conventional in nature; it is schematically represented by the lead 30 (denoted AVC) connected between the signal grid circuits of the tubes used in the network 2 (the tubes of networks 3 and Il may also be controlled) and the rectifier 3 I. The I. F. energy may be impressed on rectifier 3l, and the direct current voltage output thereof is employed to decrease the gain Vof each of the controlled tubes as the carrier amplitude increases. The AVC circuit acts to decrease the gain of the controlled tubes in a manner such that the I. F. carrier amplitude at the input of network I8 is substantially uniform, regardless of a wide varia-- tion of carrier amplitude at the collector I. The numeral 32 denotes a lter element, such as a resistor-condenser network, for suppressing pulsating components in the AVC bias. The action of the AVC is Well known; for weak signal reception the gain of each controlled tube, hence the space current flow thereof, is a maximum. As the carrier amplitude increases, the AVC bias acts to reduce the gain of each tube; the space current flow thus decreases.

The potential of the plate 8 of oscillator tube 5 will fluctuate in a fairly substantial manner because of the variation in space current flow in the controlled tubes, and which Variations are caused by the AVC bias. The variations in plate potential of plate 8 may occur to an extent sufficient to cause the operating frequency of tank circuit S-Ill to Vary in a manner such that the desired station is lost. The circuits of the oscillator tube are arranged practically to eliminate the frequency change caused by the oscillator plate potential variation. The circuits of oscillator tube 5 are a combination of the well known Hartley type of oscillator circuit and the equally Well known tuned plate circuit oscillator. By combining these types of oscillator circuit arrangements there is secured opposing frequency variation tendencies which result in a substantially constant operating frequency regardless of oscillator plate potential change.

The Hartley type of oscillator circuit employs the oscillator tank circuit in both the plate and grid circuits. The curve A in Fig. 2 shows the relation between the frequency drift and plate voltage change for this type of oscillator circuit.

It will be observed that the curve A has a falling relation between the frequency and plate voltage increase. In other words, for the Hartley oscillator circuit, as the plate voltage of the oscillator tube plate increases, the operating frequency of the circuit decreases. On the other hand, for the tuned plate oscillator circuit the frequency of the circuit increases as the plate voltage increases. The curve B in Fig. 2 shows the nature of this curve, and it is to be understood that the curve B illustrates the frequency drift-plate voltage variation of an oscillator circuit whose tank circuit is wholly in the plate circuit of the oscillator tube. It will be observed that curves A and B are opposing, and that they nullify their respective effects as the plate voltage increases. The circuits of the oscillator in Fig. 1 are a combination of the two Wellknown circuits, and it will be seen that tank circuit 9-I0 is not only in the plate and grid circuits of the oscillator tube, but the control grid 6 is reactively coupled, through coil I4, to the tank circuit. This circuit arrangement results in the production of the effects illustrated by curves A and B in Fig. 2, as the plate voltage of oscillator tube 5 increases.

The receiving circuit, therefore, will not change its operating local oscillation frequency once the variable condenser I has been adjusted to a predetermined oscillation frequency. Regardless of how the AVC circuit functions to vary .the operating plate potential of oscillator tube 5, the operating frequency of the oscillator circuit will remain substantially independent of the plate potential change. Of course, separate tubes need not be employed for the first detector and oscillator networks (a pentagrid converter tube of the 6A7 type can be employed in place thereof). As stated before, moreover, the present invention is not restricted to overcoming the frequency change effect caused by AVC action; it is of equal value to counteract such frequency change when caused by any variation in the power supply circuit which might otherwise cause the local oscillator frequency to drift from its proper value.

To illustrate the applicability of our invention to the broadcast band, as well as to the short wave range, a coil was constructed in accordance with this inventon for use on the broadcast band.. The coil was Wound on a 1 inch form with No. 30 enamelled Wire. Resistor I was 50,000 ohms; condenser I3 was 100 mmf. Coil 9 had '77 turns with tap l2 located 65 turns from the ground end. Coil I4 had 18 turns wound adjacent to the grounded end of coil 9. The stability with a type amplitude as possible in the short Wave range. In the broadcast band, the cathode tap providing maximum oscillator amplitude may result in too much oscillator voltage on the first detector, since the conversion efficiency of a rst detector suffers if there is too much oscillator amplitude, as Well as if there is too little. In any case, the value of oscillator amplitude which gives desirable conversion efficiency of the first detector will cause the frequency of a Hartley type oscillator to shift' in the manner shown in Fig. 2. Combining the tuned plate type of coupling therewith does not diminish the amplitude of oscillation,

although it does yprovide the desired frequency stability. It may be seen, therefore, that our invention permits oscillator operation in such a manner that a high degree of frequency stability and a high conversion efficiency of the first detector may be simultaneously achieved.

While we have indicated and described a systern for carrying our invention into effect, it will be apparent to one skilledl in the art that our invention is by no means limited to the particular organization shown and describedbut that many modifications may be made without departing from the scope of our invention, as set forth in the appended claims.

What is claimed is:

1. In a superheterodyne receiver of the type including a signal transmission tube, a local oscillator tube provided with a tunable tank circuit and an automatic volume control circuit arranged to vary the gain of the signal transmission tube; a common power supply circuit connected to said transmission tube and oscillator tube, and circuits electrically connected with said oscillator tube and tank circuit to introduce opposing frequency variations in said tank circuit upon a change of the oscillator tube plate potential caused by the automatic volume control action.

2. In a superheterodyne receiver of the type adapted to operate in a range of frequencies including 0.5 to 22 mc., and which receiver comprises a radio frequency amplifier, first detector and an intermediate frequency amplier; an automatic volume control circuit adapted to vary the gain of each of the signal transmission stages in response to received signal amplitude variation, a common power supply circuit connected to the various signal transmission stages; a local oscillator tube having its electrodes connected to said common supply circuit, a tunable tank circuit electrically associated With said oscillator tube, the frequency of the tank circuit increasing with plate potential increase of the oscillator tube, said plate potential increase being caused by the AVC action; and means electrically associincrease is substantially eliminated.

DUDLEY E. FOSTER. CHARLES W. FINNIGAN. 

